The present invention relates to a code division multiple access communications receiver, more particularly to a receiver for use in the base stations and mobile stations of a code division multiple access mobile communications system.
Code division multiple access (hereinafter, CDMA) is undergoing intensive development as a technique for using bandwidth more efficiently in mobile communications systems. CDMA uses a spectrum spreading process to enable multiple signals to share the same frequency band. A desired signal is extracted from the shared band by a de-spreading process.
In a direct sequence CDMA system, the different signals are distinguished by the use of different spreading codes. When a signal is extracted, the extracted signal usually contains interference from other signals, due to imperfect orthogonality of the spreading codes and other factors.
A known method of canceling this so-called co-channel interference estimates the signal received from each transmitting station on each transmission path, and subtracts, from the combined received signal, the signals estimated to have been received from stations other than the desired station. The subtractions may be performed one after another in a serial manner, or they may be performed in a parallel manner after all signals have been estimated. In estimating the received signals, it is also necessary to estimate the behavior of the radio channels, which distort the phase and amplitude of the signal on each transmission path from each station.
Accurate radio channel estimation is difficult, however, particularly in the parallel case. Therefore, co-channel interference continues to be a problem in CDMA communications.
It is accordingly an object of the present invention to reduce co-channel interference in the reception of a CDMA signal.
Another object of the invention is to improve the estimation of symbol values in the reception of a CDMA signal.
Another object of the invention is to improve the estimation of the radio channels in the reception of a CDMA signal.
The invented CDMA receiver receives a combined signal from a plurality of transmitting stations, despreads the received signal, thereby estimates the symbol values respreads the symbol values, thereby estimates the signals received from each transmitting station, and subtracts these estimated signals as estimated interference from the received signal, thereby producing a residual signal. The CDMA receiver also weights the estimated symbol value by multiplication by respective weighting factors when the estimated symbol values are respread. The weighting factors are adjusted at certain intervals.
The weighting factors are calculated so as to reduce the power of the residual signal. According to one aspect of the invention, the weighting factors are calculated from the values of the residual signal, using a mathematical technique such as the least mean square-method. According to another aspect of the invention, a hard decision is made as to the symbol values before the symbol values are weighted and respread, and the weighting factors are calculated from the differences between the estimated symbol values before the hard decision and the values after the hard decision.
The weighting factors may be used as digital filter coefficients, by convolving the symbol value or values estimated to have been received from the same transmitting station over a certain period of time with two or more weighting factors.
Besides estimating the symbol values transmitted by each station, the invented CDMA receiver preferably estimates the distortion in each transmission channel, and in estimating the signal received from each station (the estimated interference due to the station), the CDMA receiver preferably includes the estimated distortion. In this case, the weighting factors may be applied to the estimated phase and amplitude distortions, thereby indirectly applying the weighting factors to the estimated symbol values.
The CDMA receiver may combine the received signal to allow for path diversity from each transmitting station. In this case, separate weighting factors are calculated for each path, a separate estimated interference value is obtained for each path, and the estimated interference values for different paths from the same transmitting station are added together to obtain the estimated interference due to that station.
The process of estimating the symbol values and estimating and subtracting interference may be carried out in multiple stages, the residual signal output by each stage becoming the input signal to the next stage.
Subtraction of estimated interference may be carried out in either a serial or a parallel manner. In the parallel case, the estimated interference values are subtracted all at once, after the interference due to all transmitting stations has been estimated. In the serial case, the estimated interference due to each station is subtracted as soon as the estimate has been made. In the serial case, when the weighting factors are calculated from the residual signal, the residual signal from which the weighting factors for each transmitting station are calculated may be the intermediate residual signal left immediately after subtraction of that station""s estimated interference. Alternatively, the weighting factors for all transmitting stations may be calculated from the same residual signal, this being the residual signal left after all stations"" estimated interference has been subtracted.